test.cs

[HttpPost]
[Route("MergeTemplate")]
public MailMergeData MergeTemplate([FromBody] MailMergeData mailMergeData)
{
    using (var tx = new TXTextControl.ServerTextControl())
    {
        tx.Create();

        byte[] templateData = Convert.FromBase64String(mailMergeData.Template);
        tx.Load(templateData, TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        var merge = new MailMerge
        {
            TextComponent = tx
        };
        merge.MergeJsonData(mailMergeData.Json);

        byte[] documentData;
        tx.Save(out documentData, TXTextControl.BinaryStreamType.InternalUnicodeFormat);

        mailMergeData.Document = Convert.ToBase64String(documentData);

        return mailMergeData;
    }
}